The present invention relates to a method for positioning an electronic device, in which signals from satellites of a positioning system are received, and a default position is determined for the electronic device, to be used for positioning. The invention also relates to a system for positioning an electronic device, comprising means for receiving signals from satellites of a positioning system, and means for determining a default position for the electronic device, to be used for positioning. Furthermore, the invention relates to an electronic device which comprises a positioning receiver for positioning the electronic device, provided with means for receiving signals from satellites of a positioning system, and means for determining a default position for the electronic device, to be used for positioning.
Devices are known in which a positioning receiver is used for determining the location of the device. Such positioning receivers primarily rest on satellite-based positioning systems. In such positioning systems based on satellites, the positioning receiver attempts to receive a signal transmitted from satellites and containing phase-modulated information, inter alia orbital parameters of the satellites. In practical situations, however, the signal strength in the positioning receiver may be so weak, particularly indoors, that the demodulation of the signal is slow and difficult. Furthermore, the positioning can not necessarily be performed at all times.
One known positioning system is the GPS system (Global Positioning System), which comprises more than 30 satellites, of which usually a maximum of 12 are simultaneously within the sight of a receiver. These satellites transmit e.g. orbital data (Ephemeris data) as well as time data of the satellite. A receiver to be used in positioning normally determines its position by computing the propagation time of a signal transmitted substantially simultaneously from several satellites belonging to the positioning system to the receiver. For positioning, the receiver must typically receive the signals of at least four satellites within sight, to be able to compute the position.
Each operating satellite of the GPS system transmits a so-called L1 signal at the carrier frequency of 1575.42 MHz. This frequency is also indicated with 154f0, where f0=10.23 MHz. Furthermore, the satellites transmit another ranging signal at a carrier frequency of 1227.6 MHz called L2, i.e. 120f0. The carrier of the L1 signal is further modulated by navigation information at a bit rate of 50 bit/s. The navigation information comprises information about the “health” and orbit of the satellite, parameters related to the local clock of the satellite, etc. In satellites of the GPS system, e.g. so-called atomic clocks are used as the local clock.
In assisted positioning (for example, A-GPS, Assisted Global Positioning System), auxiliary information is transmitted to the receiver to accelerate the positioning. Such auxiliary data is, for example, the orbital parameters of the satellites, and the GPS time. On the basis of the auxiliary data, the receiver can faster detect and acquire the signal of a visible satellite; in other words, the time to first fix (TTFF) is reduced. One possibility to implement such a system based on auxiliary data is to use another receiver to receive signals from the satellites, to determine the data transmitted in the signals, and to transmit it to the positioning receiver. The data transmission can be implemented, for example, via a mobile communication network, wherein a receiver of the mobile communication network is arranged in connection with the positioning receiver. In such a system, the positioning receiver and the receiver used for generating auxiliary data may even be relatively far from each other. Furthermore, the validity time of the auxiliary data, i.e. the time during which the auxiliary data is still sufficiently reliable, is relatively long.
Moreover, positioning can be accelerated by determining a default position for the receiver and by transmitting it to the receiver. As the default position, it is possible to use, for example, the position of a base station close to the receiver in the mobile communication network. Consequently, this default location provides a kind of an initial guess for the position of the receiver, which accelerates the determining of the visible satellites, and the time taken for the acquisition of the satellite signals can thus be reduced. A problem in such an arrangement is, however, that it requires the transmission of auxiliary data in a mobile communication network or in another communication network, wherein the auxiliary data is not necessarily available everywhere. Furthermore, the reception of auxiliary data may be subject to a charge, which may reduce the users' interest in utilizing the auxiliary data.